The advent of a new round of stringent emissions legislation in Europe and North America is driving the implementation of new exhaust after-treatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide (NOx) emissions that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust after-treatment technologies are currently being developed that will treat NOx under these conditions. One of these technologies comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR).
Diesel exhaust fluid is known to “creep” through small gaps due to its capillary action. This requires near hermetic leak rates of gaskets on RDU's.
Mica has been used to both provide a thermal barrier to the fluid injector and facilitate a near hermetic seal with the RDU. This use of mica needs a significant force to provide a near hermetic seal. This force can be applied by either mounting bolts or a V-clamp. Prior attempts to seal with mica required large bolts or V-clamp to achieve a proper sealing force. This is neither cost effective nor packaging friendly in a tight engine bay. Other methods, such as a sealing bead, concentrate the force applied by the bolts over a smaller area but must be machined into the mounting flange or boss. This adds complexity and cost. Another method of providing a near hermetic seal with mica is complex multilayer gaskets. This again adds cost and complexity.
Another known RDU design uses an air gap as the isolator and a metal-to-metal crimp on the RDU to provide the seal. This leads to poor packaging and the air gap could potentially become clogged with urea crystals.
Yet another known RDU design uses seals on the mica by machining a sealing bead into the hub component. This concentrates the stress sufficiently to get a seal. This leads to large forces applied to the housings and thus thick robust housings must be used.
Still another known RDU design uses a multi-layer steel-mica-steel gasket with built in stress concentrators. This makes the mica gasket a costly and complex assembly.